Cell Cycle

Cell Cycle — Study Guide

1. The Big Picture: What the Cell Cycle Is

The cell cycle is the ordered sequence of events by which a cell:

1. Grows

2. Replicates its DNA

3. Divides into daughter cells

This cycle allows organisms to:

• grow and develop

• repair damaged tissues

• reproduce (in single-celled organisms)

The cell cycle must be precisely controlled so that each new cell receives a complete and accurate copy of the genome.

Genome

The entire set of genetic information contained in an organism's DNA.

In eukaryotic cells, the genome is organized into chromosomes.

2. Chromosomes and Genetic Organization

To understand the cell cycle, you must understand chromosome structure.

Chromosome

A DNA molecule packaged with proteins (primarily histones) that carries genes.

When a cell is not dividing:

• DNA exists as chromatin (loosely organized DNA).

When a cell prepares to divide:

• chromatin condenses into visible chromosomes.

Chromatid and Sister Chromatids

Before DNA replication:

• each chromosome consists of one chromatid.

After replication:

• each chromosome consists of two identical chromatids.

These are called sister chromatids.

Sister Chromatids

Two identical copies of a chromosome that remain attached at a region called the centromere.

They contain identical DNA sequences.

Homologous Chromosomes

Humans (and most animals) have chromosomes in pairs.

Each pair consists of two homologous chromosomes.

Homologous Chromosomes

Two chromosomes that:

• are similar in size

• carry the same genes

• have genes in the same locations

However they may carry different versions of those genes.

Alleles

Different versions of the same gene.

Example:

Gene: eye color
Allele 1: brown
Allele 2: blue

Thus homologous chromosomes:

• carry the same genes

• but may carry different alleles.

3. Haploid vs Diploid Cells

Haploid (n)

A cell containing one set of chromosomes.

Human haploid cells:

• 23 chromosomes

Haploid cells are typically gametes (sex cells).

Examples:

• sperm

• egg

Diploid (2n)

A cell containing two sets of chromosomes.

Human diploid cells:

• 46 chromosomes

• 23 homologous pairs

Most body cells (somatic cells) are diploid.

4. The Four Stages of the Cell Cycle

The cell cycle consists of four main stages.

Interphase

The first three stages (G1, S, G2) together are called interphase.

Interphase is when the cell:

• grows

• duplicates organelles

• replicates DNA

Cells spend most of their life in interphase.

5. DNA Replication (S Phase)

DNA replication occurs during S phase.

The purpose:

To produce two identical copies of DNA so each daughter cell receives one.

6. Why DNA Replication Is Called Semi-Conservative

Each new DNA molecule contains:

• one original strand

• one newly synthesized strand

Thus each daughter molecule conserves one parental strand.

This is called semi-conservative replication.

7. Origin of Replication

DNA replication begins at specific locations on DNA called:

Origin of Replication

A specific DNA sequence where replication begins.

Replication then proceeds outward in both directions.

8. Visualizing DNA Replication

Imagine DNA as a twisted ladder (double helix).

The process begins by unzipping the ladder.

Then each side becomes a template for building a new strand.

The result:

Two identical DNA molecules.

9. The Four Key Enzymes in DNA Replication

DNA replication involves several enzymes that each perform specific tasks.

1. Helicase

Function:

• unwinds the DNA helix

• separates the two strands

It breaks hydrogen bonds between base pairs.

Result:

DNA forms a replication fork, a Y-shaped structure where replication occurs.

2. Primase

Function:

Adds short RNA primers to the DNA template.

RNA Primer

A short RNA segment that provides a starting point for DNA synthesis.

This is necessary because DNA polymerase cannot start DNA synthesis from scratch.

3. DNA Polymerase

Function:

Builds the new DNA strand.

It does this by adding nucleotides to the growing strand.

Important Limitations of DNA Polymerase

1. Cannot start a new strand

2. Can only add nucleotides to the 3′ end

This means synthesis occurs only in the 5′ → 3′ direction.

Understanding the Direction (Visualization)

DNA strands run antiparallel:

One strand:

5′ → 3′

Other strand:

3′ → 5′

DNA polymerase can only build:

5′ → 3′

Therefore replication occurs differently on the two strands.

10. Leading and Lagging Strands

Because DNA strands run opposite directions, replication proceeds in two ways.

Leading Strand

The strand that is synthesized continuously.

This occurs because DNA polymerase can follow the replication fork directly.

Thus:

• replication proceeds smoothly

• no interruptions

Lagging Strand

The strand synthesized in fragments.

Because polymerase must move away from the fork.

Thus replication occurs in short segments called:

Okazaki Fragments

Short DNA segments produced on the lagging strand.

DNA Ligase

Ligase connects the fragments together.

Function:

• seals breaks in the sugar-phosphate backbone

• produces a continuous strand.

11. Mitosis

Mitosis is the process of nuclear division.

It produces two genetically identical nuclei.

After mitosis, cytokinesis divides the cytoplasm.

Result:

Two identical daughter cells.

12. Stages of Mitosis

Key Structures in Mitosis

Mitotic Spindle

A structure made of microtubules.

Function:

• moves chromosomes during cell division.

Centromere

The region where sister chromatids are attached.

The spindle attaches here via kinetochores.

13. Cytokinesis

Occurs after mitosis.

Cytokinesis

Division of the cytoplasm into two cells.

Result:

Two separate daughter cells.

14. Meiosis

Meiosis is a specialized form of cell division that produces gametes.

Gametes

Haploid reproductive cells.

Examples:

• sperm

• egg

15. Comparison of Mitosis and Meiosis

16. Two Unique Events in Meiosis

Two mechanisms generate genetic variation.

Crossing Over

Occurs during Prophase I.

Homologous chromosomes exchange segments of DNA.

Result:

New combinations of alleles.

Independent Assortment

Occurs during Metaphase I.

Homologous chromosome pairs align randomly.

This produces many possible combinations of chromosomes in gametes.

17. Genetic Consequences

Both processes increase:

Genetic Diversity

Meaning:

Offspring receive unique genetic combinations.

18. Chromosome Separation in Meiosis

The two divisions separate different structures.

Anaphase I

Homologous chromosomes separate.

Sister chromatids remain attached.

Anaphase II

Sister chromatids separate.

This resembles mitosis.

19. Nondisjunction

Sometimes chromosomes fail to separate correctly.

Nondisjunction

An error in chromosome separation during meiosis.

Result:

Gametes with abnormal chromosome numbers.

Example consequences:

• Down syndrome

• other chromosomal disorders.

Professor Questions (with Answers)

1. Write the 4 stages of the cell cycle.
   Answer: G1, S, G2, M

2. A haploid cell has set of chromosomes. A human haploid cell has chromosomes.
   Answer: 1 set, 23

3. A diploid cell has sets of chromosomes. A human diploid cell has chromosomes.
   Answer: 2 sets, 46

4. What is the difference between sister chromatids and a homologous pair?
   Answer: Sister chromatids are identical copies of the same chromosome joined at a centromere, while homologous chromosomes are a pair of similar chromosomes (one from each parent) that carry the same genes but may contain different alleles.

5. How are homologous pairs of chromosomes similar?
   Answer: same size, carry the same genes in the same order

6. How are homologous pairs different?
   Answer: they can carry different forms of a gene called alleles

7. In which stage does DNA replication occur?
   Answer: S phase

8. What happens to chromosomes during DNA replication?
   Answer: they change from one chromatid to two attached sister chromatids

9. Why is DNA replication called semi-conservative?
   Answer: each new DNA molecule contains one parental strand and one new strand

10. What term describes the location where replication begins?
    Answer: origin of replication

11. What are the four enzymes in DNA replication and their functions?
    Answer: helicase (unzips DNA), primase (adds RNA primers), DNA polymerase (adds nucleotides), ligase (joins fragments)

12. What are two limitations of DNA polymerase?
    Answer: cannot start a new strand, can only synthesize 5′ → 3′

13. Which strand is made continuously?
    Answer: leading strand

14. Which strand is made in fragments?
    Answer: lagging strand

15. Mitosis stage table
    Answer:

16. What are the cells produced by meiosis called?
    Answer: gametes

17. Compare mitosis and meiosis.
    Answer: mitosis produces 2 identical diploid cells; meiosis produces 4 genetically different haploid cells

18. What are two events unique to meiosis?
    Answer: crossing over and independent assortment

19. What is the consequence of those events?
    Answer: increased genetic diversity

20. What separates in anaphase I?
    Answer: homologous chromosomes

21. What separates in anaphase II?
    Answer: sister chromatids

22. What is the term for failure of chromosomes to separate properly?
    Answer: nondisjunction